1. Field of the Invention
The present invention relates to an apparatus and a method for measuring the temperature of an object heated in a Rapid Thermal Processing (RTP) system. More specifically, the present invention discloses a method and apparatus for measuring the temperature of an object if interference effects disturb the conventional pyrometric methods of temperature measurements. The present invention is particularly useful in the case of semiconductor wafers which have layers of material deposited on the back side of the wafer, and in the case of Rapid Thermal Chemical Vapor Deposition (RT-CVD). The starting point of the present invention is that stray light from the heating lamps entering the measurement pyrometer aperture raises the background signal and lowers the measurement accuracy of the conventional pyrometers, and the present invention shows a new method and an apparatus for reducing the stray light.
2. Description of the Prior Art
The field of rapid thermal processing has been chiefly concerned with uniformly heating semiconductor wafers to a known temperature, and measuring and controlling the temperature time profile of the wafer for the various annealing, chemical reaction, and film growth techniques required by the various processes. To achieve uniformity of heating, the individual lamps of the heating systems have been separately controlled by the control system, and the reflector systems have been carefully designed to irradiate the wafer in a known pattern. To this end, the reflector systems have made use of diffuse reflectors to homogenize the light intensity incident on the wafer, as outlined in "Modeling of Three-Dimensional Effects in Rapid Thermal Processing of Eight Inch Wafers", K. L. Knutson, S. A. Campbell, and F Dunn, IEEE Transactions on Semiconductor Manufacturing, 7, (1994) p. 68-71.
The pyrometric measurement of wafer temperature in RTP systems is known to be complicated because the relatively weak thermal emission from the wafer must be filtered out from the relatively much stronger background radiation from the much hotter heating lamps. The pyrometer generally measures radiation at a wavelength where the radiation from the lamps reaching the wafer surface is minimal. The accuracy of the measurement is fundamentally determined by the selectivity of the filter used to determine the wavelength. The background radiation from the RTP system lamps can come either from reflection of the radiation from the wafer, or from transmission of radiation through the wafer.
Optical pyrometric temperature measurements in RTP systems require the selection of a wavelength and an optical bandwidth of the measurement. This choice is made on the basis of the material parameters of the wafer, the reactor chamber, the reflectors, and the radiation sources. (See for example U.S. Pat. No. 5,188,458 and DE 4012615C2)
The so called "Ripple Technique" (U.S. Pat. No. 5,154,512) is a special case of optical pyrometric measuring. The lamp power supply is modulated with a frequency of 5-120 Hz, and the emission from the wafer is measured with a first optical fiber while the emission from the lamps is measured with a second optical fiber. The wafer temperature usually cannot change fast enough to follow the modulation frequency, while the lamps in general can. The variation in lamp radiation is measured with the second optical fiber, and the variation of the radiation measured with the first fiber can be used to determine the radiation reflected from the wafer. Once the reflection coefficient for the wafer is known for a particular wavelength, the emissivity of the wafer at that wavelength is known, and the unvarying part of the radiation collected by the first optical fiber due to the emission from the wafer can be used to calculate the temperature. This technique is, however, very sensitive to temperature and the arrangements for the necessary mechanical adjustments of the light fibers are very expensive.
Use of optical fibers and light pipes to collect the light does not change the principles of the measurements cited above.
The best signal to background ratios that can be reached using monochromatic or narrow bandwidth pyrometric techniques is about 1000:1 (DE4012615C2), which allows good reproducibility of the temperature measurement. However, such monochromatic or narrow bandwidth pyrometric measurement can be very strongly influenced by layers of material of different indices of refraction on the surface of the object being measured, and the method is practically unusable if the measured wafer surface has optically thin layers of variable layer thickness deposited thereon. The reflection coefficient at any wavelength, and hence the emissivity (which is 1 minus the reflectivity) of the surface, varies enormously as the various layer thicknesses vary. For example, as one layer of silicon dioxide grows from 0.25 micron thickness to 0.5 micron thickness on a silicon wafer, the reflectivity for 1.5 micron light varies from a minimum of 6% to a maximum of 42%. In such cases, a pyrometer can only be used if the bandwidth of the measured radiation is broad enough that the constructive and destructive interferences within the measuring bandwidth compensate each other. In the case cited above, a wavelength of 3 micron would have a minimum in reflectivity as the thickness of the oxide varied when the 1.5 micron light has a reflectivity maximum. A measurement band of from 1.5 to 3 microns, (or a wavelength of 2.25.+-.0.75 microns) would be preferable to compensate the variations in emissivity. In such a case, it can be shown (on conservation of energy grounds) that the broadband wavelength filter cannot filter out the radiation from the background, and the signal to background ratio becomes very small.
We have recognized that, under specially determined conditions, it is possible to effectively separate the photons thermally emitted from the wafer from the photons from the heating lamps which have been reflected from and transmitted through the wafer by using the wave vector or the angle which the photons leave the surface. Using a statistical ray tracing procedure, we have calculated the angular distribution of lamp radiation reflected from the surface of a specularly reflecting wafer in the cases of an ideal rectangular reflector chamber and a cylindrical reflector chamber with specularly reflecting surfaces, and have found that, under special conditions, relatively little lamp radiation reflected from the surface is normal to the surface. The thermal radiation, however, obeys Lambert's law, and is mostly normal to the surface. The two types of radiation can thus be separated by controlling the acceptance angle of the pyrometer.